Buildings require exit signs throughout hallways, corridors, and other areas of the building to indicate the locations of emergency exits. To be effective, exit signs must remain continually illuminated and are designed to maintain a specific minimum luminance over a defined lifespan. Yet, the light source's intensity degrades over time. To overcome this, the light source for the exit sign usually start illuminating at a higher level at the start of its life so that a longer amount of time can pass before the light source needs replacement.
Exit signs can use a variety of light sources, including incandescent lamps, light emitting diodes (LED), or electroluminescent (EL) style elements, such as an EL panel or light emitting capacitor (LEO). An EL-style element essentially operates as a capacitor having a dielectric layer and a phosphor layer disposed between two conductive electrodes. The front electrode is typically transparent. When an alternating electric current is applied to the two electrodes, an electromagnetic field is developed, and the phosphor layer emits light.
Details related to electroluminescent elements are provided in U.S. Pat. Nos. 5,662,408; 5,816,682; and 7,191,510, which are incorporated herein by reference in their entireties. For example, the transparent front electrode can be made out of indium tin oxide. The phosphor layer has encapsulated phosphor screen-printed over the front electrode. The dielectric layer can contain a solvent, a binder, and barium titanate particles that are screen-printed over the phosphor layer. The rear electrode typically has a solvent, a binder, and conductive particles such as silver or carbon that are screen-printed over the dielectric layer.
The EL-style elements operate with relatively little current, which makes them well suited for light sources that operate continuously. Therefore, exit signs used in buildings and the like have been developed that use electroluminescent elements. Because exit signs form such an important part of a building's emergency system, enhanced capabilities of an exit sign can be a great advantage to maintenance, security, and emergency personnel.